He's realizing that he's no longer just a neutral observer now. Whether he likes it or not, he has failed to stay uninvolved and Anna's abrupt departure from his daily routine is bothering him because he really is involved now.
This chapter is all about biting the bullet and I like it, brutal honesty is sometimes necessary to break the stale status quo. For a moment, I feared he would ask Anna to continue repaying her debt like before, but the most he did was express gratitude for the fun time. Turn the page and move on, yes it was fun... but as a friendship it was a farce.
Even though it wasn't his place to say it, I really like that he came clean to her oblivious childhood friend about Anna's feelings. Neither the clueless Sosuke nor the cowardly Anna will break this stale state of affairs and allow everyone (well, mostly her) to move forward, so it falls to an outsider like Kazuhiko to break it, even if it means doing something like this (and he might get vilified in the process).
Even after her defeat Anna still continues to run away from her feelings and hide it, but this corners her and she can't run away anymore. The cat is out of the bag. Assuming no "I didn't hear anything" or "He's lying!" cop outs, this will force her to face her feelings and, while painful, will help her move on. Maybe the saying holds true: the truth will set you free.